Since the mid-1980s metallocene catalysts have been used in high-pressure reactors—mainly for producing ethylene-backbone polymers (polyolefins where ethylene is the principal monomer), including ethylene copolymers with co-monomers including one or more of propylene, butene, and hexene, along with other specialty monomers such as 4-methyl-1,5-hexadiene. For example, U.S. Pat. No. 5,756,608 to Langhausen et al., reports a process for polymerizing C2 to C10 1-alkenes using bridged metallocene catalysts. Until recently, polypropylene production under homogeneous supercritical conditions has been seen as impractical and unworkable, due to a lack of catalyst systems that could produce commercially useful polypropylene at temperatures much above the critical temperature and above the solid-fluid phase transition temperature. However, a process to produce commercially useful polypropylene (and other related polymers) in a high pressure system has now been disclosed in WO2004/026921, which provides advantages, such as increased catalyst productivity, higher throughput, shorter residence times, etc. Likewise, new propylene-based polymers with tailored composition and/or molecular weight distributions were disclosed. Thus there is now a need in the art to develop new processes capable of greater economy and efficiency in the separation of the polymers of this new process from the low molecular weight components of the polymerization process, and, preferably, of economically recycling those low molecular weight components to the polymerization system feed.
WO 1993/11171 discloses a polyolefin production process that comprises continuously feeding olefin monomer and a metallocene catalyst system into a reactor. The monomer is continuously polymerized to provide a monomer-polymer mixture. Reaction conditions keep this mixture at a pressure below the system's cloud point pressure, and thus create a two-phase reaction mixture consisting of a polymer-rich and a monomer-rich phase. The reaction temperature is also maintained above the polymer's melting point.
WO 1992/14766 discloses a process comprising the steps of (a) continuously feeding olefinic monomer and a catalyst system, with a metallocene component and a cocatalyst component, to the reactor; (b) continuously polymerizing that monomer in a polymerization zone reactor under elevated pressure; (c) continuously removing the polymer/monomer mixture from the reactor; (d) continuously separating monomer from molten polymer; (e) reducing pressure to form a monomer-rich and a polymer-rich phase; and (f) separating monomer from the reactor. WO 1992/14766 focuses on catalyst killing systems for ethylene-rich polymers and neither discloses single-phase polymerization conditions for producing commercially useful propylene-based polymers, nor describes how to obtain the economic benefits of low energy consumption and low plant investment via fluid-liquid phase separation of propylene-based polymers under supercritical conditions.
WO2004/026921 discloses a process to polymerize a range of olefins having three or more carbon atoms, and optionally a wide range of olefin and/or diolefin comonomers, with a catalyst compound (such as a metallocene), activator, and optionally diluent or solvent, at a temperature above the crystallization temperature of the polymerization system and a pressure no lower than 10 MPa below the cloud point pressure of the polymerization system, where the polymerization system comprises any comonomer present, any diluent or solvent present, the polymer product, where the olefins having three or more carbon atoms are present at 40 weight % or more. Thus, WO2004/026921 teaches a polymerization process for making commercially useful propylene-rich (or higher alpha-olefin-rich) polymers under a wide range of supercritical operating conditions, including both single and two-phase reaction systems, with or without use of an inert solvent component, where the polymer is in solution, i.e. is dissolved in either the single phase reaction medium, or predominantly in one of the two phases in a two phase system. The current invention presents a process for the efficient and economical separation of the polymer from the lower molecular weight components of the reaction medium and for economical recycle of the lower molecular weight components to the polymerization system feed, useful in the polymerization system described in WO2004/026921.
U.S. Pat. No. 6,881,800, U.S. Pat. No. 7,163,989 and W02002/034795 (by Friedersdorf) describe a polymerization system for a wide variety of polyolefin polymers, including propylene-rich polymers, with metallocene catalyst systems. However, because the systems described are solvent based systems, the polymerization system pressure range that is disclosed is from 7.5 to 20 MPa, which is below the range of the current invention (20 to 200 MPa).
Despite these advances in the art, there remains a need for polymerization processes that provide commercially useful propylene-rich polymers, especially those having high molecular weight and high crystallinity, prepared at higher temperatures and productivities than otherwise possible, while providing for the economic separation of monomer from polymer, i.e. with low capital investment and low energy consumption, thus affording advantaged product separation and monomer recycle.